The present invention relates to the field of video image processing, and in particular to an apparatus and method of displaying two or more video images on a display such as for example a television device.
In video display devices such as a television, flicker is removed from the pictures displayed on a screen in that the picture sequences, received from the video sources with a given display frequency, are displayed on the screen with twice this display frequency. The picture sequences delivered by the video sources are present especially as sequences of first and second half pictures, which are displayed on a screen in respective line-interleaved rasters.
A picture consisting of half pictures is displayed without flicker line-by-line in a reproduction raster, such that, in a periodic sequence, the odd picture lines, the so-called α raster, and the even picture lines, the so-called β raster, are each scanned twice in immediate succession. With the so-called AABB transformation of a 50 Hz television picture, a first half picture is displayed in the α raster twice in immediate succession and a second half picture is similarly displayed in the β raster. The displayed picture thus appears flicker-free to the human eye, with a frequency of 100 Hz.
From the DE 195 34 781 C1, a method is known for so-called picture-in-picture superimposition, such that a picture delivered from a video source is to be displayed on a small scale in a main picture. With this method, the superimposed picture is present as a half picture sequence with a reproduction frequency of 50 Hz. The second picture is present as a half picture sequence with twice the reproduction frequency, that is 100 Hz, each half picture of the main picture always being made available twice in immediate succession. To double the reproduction frequency of the superimposed picture, the first and second half pictures are always written into a half picture memory and are read out twice per write process. The timing of the read process, especially the read-start, is dependent on the display of the main picture.
Since the superimposed picture and the main picture are not made available synchronized, the problem can arise that while a half picture of the superimposed picture is being written into a half picture memory, a read process takes place that overtakes the write process. In the picture that is superimposed on the main picture, picture portions then become visible which were written into the memory in two different, that is temporally distant, write processes. Moving pictures show a troublesome displacement between these picture portions, which is called a “joint line.” To prevent such a joint line, the known methods evaluate the phase shift between the superimposed picture and the main picture and, depending on the measured phase shift, it is decided whether the first half pictures or the second half pictures of the superimposed picture are to be displayed in the α raster and the respectively other half pictures in the β raster. Here one utilizes the circumstance that, for some phase shifts, the read pointer can overtake the write pointer during read-out from one of the half picture memories while, with this phase shift, such an overtaking effect cannot occur during the read-out from the respectively other half picture memory. However, there are phase shifts between the superimposed picture and the main picture for which a joint line appears when reading out the superimposed picture from the picture memory, both when displaying the first half picture in the α raster and when displaying the second half picture in the α raster. The known method thus cannot prevent joint lines entirely.
Other known methods to prevent such joint lines utilize larger memories to prevent the read pointer from overtaking the write pointer. However, these methods presuppose not only more complex memories but also more complex memory space management.
Therefore, there is a need for an apparatus and method for the simultaneous display of a first picture sequence and a second picture sequence that is free of joint lines.